-The math says it's a 1.3333333 difference (8 divided by 6 = 1.3333333) which means that you are sending 1.33333333 more pulses to the tach than you should be.
-so 6000 times 1.3333333 = 8000rpm.
-----------------------------------------
-I just happen to have a tachometer on my ignition machine that has the 4-6 & 8 switch so I tested an 8 cylinder at 6000 rpm and the tachometer when set on 6 cylinder does indeed read close enough to 8000 to believe the math (if you use all seven 3's).